CCD imagers have been known in which charge transfer from all the charge transfer channels in an image register has been in one direction during alternate field scans and in the opposite direction during the intervening field scans. Y. Takemura in U.S. Pat. No. 3,777,061 issued Dec. 4, 1983 and entitled "Solid State Image Pickup Device" describes a field transfer type of CCD imager in which the entire content of the image register is alternatively transferred, either across a first line register (side-loading and side-unloading) to a first field storage register at one end ofthe image register, or to a second field storage register at the other end of the image register, between the image register and a second line register. The field of charge packets descriptive of image elements stored in the first field storage register is transferred a line at a time to the first line register during line retrace intervals in odd-numbered fields, side-loading the first line register with charge packets parallelly-in-time with each other; and the first line register is subsequently operated as a shift register to unload these charge packets from an end of the register serially-in-time during line trace intervals in the odd-numbered fields, for conversion to samples of video signal. The field of charge packets descriptive of image elements stored in the second field storage register is transferred a line at a time to side-load the second line register during line retrace intervals in even-numbered field; and the second line register is subsequently operated as a shift register to unload these charge packets from an end of the register serially-in-time during line trace intervals in the even-numbered fields, for conversion to samples of video signal.
A CCD imager may have its image register partitioned into halves with charge packets descriptive of elements in the top or left half of the image transferred out of the image register in a direction opposite to charge packets descriptive of elements in the bottom or right half of the image. U.S. patent application Ser. No. 562,528 filed Dec. 19, 1983 by H. Elabd and W. F. Kosonocky, entitled "Imagers with Registers Partitioned for Simultaneous Charge Transfers in Opposing Directions" and assigned to RCA Corporation describes such CCD imagers. Other such CCD imagers are described in U.S. patent application Ser. No. 590,888 filed Mar. 16, 1984 by H. Elabd and W. F. Kosonocky, entitled "High Resolution CCD Imagers ", and assigned to RCA Corporation.
U.S. patent application Ser. No. 646,103 entitled "Opposite-Direction Multiple-Phase Clocking in Adjacent CCD Shift Registers" filed by the present inventor Aug. 31, 1984 describes how every other one of a plurality of parallelled charge transfer channels may be clocked (i.e., alternate ones of the channels) so as to transfer charge in a direction opposite to charge transfer in the remainder of the charge transfer channels.
The transfer of charge from the image register of a CCD imager is carried out in this way in the present invention. This facilitates the solution of several longstanding problems encountered in CCD imager design and operation, particularly in CCD imagers in which photoconversion of a radiant energy image to charge packets sampling elements of the image takes place within the charge transfer channels of the image register themselves. A longstanding problem in CCD imagers, particularly of the field transfer type, is the phenomenon known as "transfer smear". U.S. Pat. No. 4,010,319 issued to P. A. Levine on Mar. 1, 1977, entitled "Smear Reduction in CCD Imagers", and incorporated herein by reference describes how this undesirable smear is generated. While the problem of transfer smear has been specifically solved for static images where high brightness areas do not recur in the same portions of line scan on separated scan lines, the problem has not herebefore been generally solved. Certain other aspects of the transfer smear problem have been solved, but the problem has remained intractable to general solution.
Another longstanding problem in CCD imagers of field transfer type is field shading owing to dark current integration. U.S. Pat. No. 4,496,982 issued Jan. 29 1985 to P. A. Levine, entitled "Compensation Against Field Shading in Video From Field-Transfer CCD Imagers", and assigned to RCA Corporation provides a primer with regard to this problem and is incorporated herein by reference.
In most CCD imagers a parallel-in/series-out CCD line register is used to convert groups of charge packets received parallelly-in-time for supplying those charge packets serially-in-time for subsequent conversion to a continuous video signal. The groups of charge packets parallel-in-time are supplied to the CCD line register from spatially adjacent and parallel charge transfer channels. The CCD line register is side-loaded under gate electrodes which must be interleaved amongst other gate electrodes. These other gate electrodes are needed to implement the normal CCD charge transfer processes in the shift register operation which takes place after side-loading and provides for putting the charge packets out serially-in-time. There are practical problems with obtaining enough space on the surface of the CCD substrate in which to fit these other electrodes, it being desirable to make the gate electrodes under which side-loading of charge packets takes place to have lengths long enough to span the respective widths of the charge transfer channels from which the charge packets are transferred during side-loading. The invention provides relief from the output-line-register gate-electrode-crowding problem.